This invention relates to a coil launcher and, more particularly, to an improved drive circuit which energizes the coils of the launcher successively and efficiently to accelerate a projectile through the launcher.
Projectile launchers using electromagnetic forces to accelerate a projectile rather than conventional projectile launchers which rely primarily upon gun powder have been proposed heretofore. Typical of one type of launcher is U. S. Pat. No. 4,718,322 in which parallel rails are energized by power supplies for exerting electromagnetic forces on a projectile that travels along the rails. These so-called rail launchers require extraordinarily high currents, on the order of thousands if not millions of amperes, to accelerate a projectile along the rails. Such high currents present an inherent drawback to the practical utility of a rail launcher.
Another type of launcher has been described as a coil gun, an early example of which is described in U. S. Pat. No. 2,235,201, and further improvements to the coil gun are described in U. S. Pat. No. 3,611,783. Coil guns of the type described in these patents are comprised of sequentially energized induction coils. The presence of the projectile as it moves through the barrel of the coil gun is sensed, as by an optical sensor, thereby controlling the deenergization of one coil and the energization of the next-following coil to accelerate the projectile along the barrel. Although the requisite current levels needed to operate the coil gun are less than those needed to operate a rail launcher, relatively complicated coil drive circuits have been proposed heretofore to achieve successive coil energization.
A proposal for an efficient and practical power conditioner, or drive circuit, for a coil gun is described in the paper entitled "Power Conditioner for a Coil-Gun", authored by Zabar et al. and presented at the Sixth IEEE Pulsed Power Conference in Arlington, Virginia, June 29-July 1, 1987. In the coil gun described in this paper, a barrel is segmented into sections formed of sets of coils. Each section is formed of plural phases, such as three phases, and is sequentially energized, thereby producing a traveling "wave packet" of flux density. A projectile placed in the barrel of the coil gun is provided with a conductive sleeve in which azimuthal currents are induced by the change in flux density. As a result, a force proportional to the product of the induced currents and the flux density is exerted on the projectile and moves axially with the traveling wave packet of flux density. In addition to the axial force, a radial force is exerted on the projectile to keep it positioned on the axis of the air gap included in the barrel, thereby avoiding friction between the projectile and the barrel wall.
Advantageously, the projectile used in this proposal can be provided with discrete coils instead of one cylindrical sleeve.
The aforementioned Zabar et al. paper recognizes that all of the plural-phase sets of coils along the barrel can be excited simultaneously to generate the traveling flux wave. To accelerate the projectile, the frequency of energization of the coils may be constant or, for the sake of efficiency, may increase as the projectile advances from the breech to the muzzle of the gun. However, a suitable power supply having a variable frequency generator adequate to accelerate the projectile is not practical. This is because the frequency of the current supplied to the coils must be increased by at least one order of magnitude as the projectile moves along the length of the barrel. Furthermore, since those coils that are not proximate the projectile during its traversal of the barrel do not generate any significant accelerating force, the energy supplied thereto is wasted until the projectile moves within range.
The power conditioner proposed in the Zabar et al. paper overcomes these deficiencies and disadvantages by segmenting the barrel into sections consisting of sets of coils, by grouping the coils into phase windings in which a number of coils may be connected in series or in parallel, and by connecting a separate power supply to each phase winding of each section. As described therein, each power supply includes a pre-energized capacitor connected in series with a respective phase winding of a respective section. As an example, if ten sections of 3-phase windings are used, a capacitor is connected to each phase in each section, thus resulting in thirty such capacitors. With reference to one phase, switches, such as solid-state switches (e. g. thyristors) are connected in series with the ten phase windings, respectively. When a switch is closed, the phase winding coupled thereto is connected into a series circuit with a capacitor, thereby forming an LC circuit whose resonant frequency is proportional to 1.sqroot.LC, wherein L is the inductance of the phase winding connected by the switch and C is the capacitance of the capacitor connected thereto. Since the capacitor is pre-charged, it discharges through the coil to produce an oscillating current whose resonant frequency is proportional to 1.sqroot.LC. The current through the coil generates a magnetic flux which, in turn, produces an axial force on the projectile to accelerate it. As a result of the transfer of energy from the coils to the projectile and of the resistive losses in the coils and in the projectile sleeve, the amplitude of the coil current decays with time.
After a few cycles of the coil current, the projectile has, of course, moved along the barrel; each of the phase windings of the barrel section that had been connected to the pre-charged capacitor is disconnected; and the corresponding phase winding of the next section is connected by its switch not only to the capacitor associated with it but also to the previous capacitor. As a result, two capacitors now are connected in series; and the pre-charged voltage across the newly connected capacitor is added to the remaining voltage present across the next preceding capacitor which had been connected in series with the preceding phase winding. It follows that none of the energy initially stored in the capacitor remains unutilized.
As pointed out in the Zabar et al. paper, if the pre-energized capacitors are charged to the same voltage V.sub.0 with alternating polarities and the switches are controlled such that the disconnection of one coil and the connection of the next occurs at the time that the decaying current passes through a zero reference level, then, the energy transferred to the projectile through a phase winding increases. Furthermore, as the capacitors are connected successively in series, the overall capacitance decreases so as to increase the resonant frequency of the coil current. Consequently, the linear velocity of the traveling wave packet of flux density also increases to continue the acceleration of the projectile.
Thus, as described in the Zabar et al. paper, by successively increasing the number of capacitors connected in series with each successively energized phase winding, a relatively simple power conditioner is provided to generate increasing voltages at increasing frequencies through successive, individual phase windings for accelerating the projectile until a relatively high muzzle velocity is attained.
It has been found, however, that proper design of a coil gun of the type proposed in the Zabar et al. paper suggests that the overall length of a barrel section and, thus, the number of coils in the phase winding, should be proportional to the average projectile velocity in that section. Since the projectile exhibits the lowest velocity in those sections adjacent the breech end of the barrel, such sections would be too short to be efficient in accelerating the projectile.
Therefore, and in accordance with the present invention, those sections at least at the breech end should be merged. Each merged section should be supplied by a stepwise increase in frequency to match the frequencies of the sections as originally designed.